1. Field of the Invention
The present invention relates to a baked color pencil lead which is excellent in mechanical strength such as flexural strength while having a vivid coloring property and sufficient intensity of the drawn lines and which can readily be erased with an eraser, and which is suited particularly to a color pencil lead for a mechanical pencil, and to a process for producing the same.
2. Description of the Related Art
Conventional baked color pencil leads have been produced by kneading and extrusion-molding a blending composition comprising a filler such as boron nitride and a binder such as clay and subjecting a molded article to heat treatment to prepare a porous lead and filling pores of this lead with an ink comprising a dye and the like.
In this case, important characteristics required for leads are high mechanical strength, a vivid coloring property, high intensity of drawn lines, good aging stability and easy erasability with an eraser.
In this regard, an ink to be filled has to be non-volatile in terms of intensity of drawn lines and aging stability.
If a volatile ink is used, a remaining dye is solidified, so that the lead is felt hard and scratching when used to write and the intensity of the drawn lines becomes low. When only the solidified dye finally remains, the dye has difficulty in coloring, so that even a hue of the lead changes.
On the other hand, in the case that a non-volatile ink is filled, a dye dissolved in a high boiling point organic compound having a low vapor pressure is filled into pores of a lead. However, inks having satisfactory characteristics in terms of solubility of a dye, writing performance and erasability with an eraser have not yet been obtained.
In order to solve these problems, solvents for non-volatile inks to color leads are disclosed in Japanese Patent Publication No. Sho 58-5951, Japanese Patent Publication No. Hei 4-23671 and Japanese Patent Application Laid-Open No. Hei 7-41723.
However, many of these solvents are solid or viscous liquid at room temperature, so that the erasability with an eraser is not satisfactory. Further, in order to assure strength of a lead, a size of the pores should not be increased so much, and it is difficult to fill an ink using a solid or a viscous liquid as a solvent into a lead having fine pores. As a result, produced is the problem that a difference occurs between concentrations and hues at the head and the core of the lead. Further, the solvents which are liquid at room temperature are included in the publications described above, but they are not satisfactory in terms of solubility of a dye and color intensity of the drawn lines.
In particular, it is described in Japanese Patent Publication No. Sho 58-5951 that polyalkylene glycol alkyl ether is used for the purpose of facilitating to allow a dye ink to be impregnated. However, a specific kind and a using amount thereof are not specified, and the ether is not used as a principal component but used as an additive.
Further, disclosed in Japanese Patent Publication No. Sho 64-2155 is a baked type pencil lead filled into pores with a polyoxyethylene alkyl ether base nonionic surfactant together with a mineral oil for the purpose of providing an antistatic effect. However, the baked type pencil lead produced by this process is a so-called black pencil lead comprising a binder comprising clay and various organic substances and a colorant such as graphite as principal components. It is different from a color pencil lead prepared by filling a dye ink into a white or light-colored porous baked lead comprising a colorless or white filler and a binder.
Further, disclosed in Japanese Patent Application Laid-Open No. Sho 59-218896 is a process for producing a color lead by impregnating pores of the lead with a high penetrable ink containing lower alcohol such as ethanol, benzyl alcohol or toluene as a medium and then drying to remove the medium described above, followed by impregnating the lead with a non-volatile ink containing a fatty acid such as oleic acid or a fatty acid ester as a medium. However, this production process comprises the steps of impregnating the pores with a high penetrable ink, then drying to remove the medium such as benzyl alcohol and impregnating the lead with a non-volatile ink. That is, two or more impregnation steps such as impregnation step.fwdarw.drying step.fwdarw.impregnation step - - - are required, and therefore there is a problem that the production steps are complicated and color shade is caused by impregnation of the ink carried out twice.